This invention relates to television kinescope deflection circuit devices and arrangements for improving the S-correction.
In television kinescopes or picture tubes in which an electron beam is deflected by a magnetic field generated by the flow of the deflection current in a deflection winding, certain nonlinearities of the generated raster appear. So-called "linearity" distortion results from the changeover at center of scan from damper diode to trace switch, and is corrected by a saturable reactor acting on only one side of scan. Side pincushion distortion requires a correction at both right and left sides of the raster by a device coupled in the horizontal circuit but controlled at the vertical deflection rate.
Another type of nonlinearity is attributable to the fact that the effective center of deflection of the electron beam is not coincident with the center of radius of curvature of the screen of the kinescope. The distance from the center of deflection to the edges of the kinescope faceplate is greater than the distance to the center of the kinescope faceplate or screen. This difference in distance tends to be greater in the case of kinescopes having large deflection angles, and also increases as the radius of curvature of the faceplate increases (as the faceplate becomes flatter).
As a result of this source of nonlinearity, the deflection of the electron beam as a function of time becomes progressively greater near the edges of the screen, as seen on the raster displayed on the screen of the kinescope. Since the information to be displayed in a television system is encoded at a constant rate, the information as displayed on the screen will appear to become compressed in the center of the raster and stretched or expanded near the edges.
A common method of compensating for the nonlinearity is to place an "S" capacitor in series with the deflection windings. With the addition of the S-capacitor, the deflection current rather than being a substantially linear ramp during the recurrent scanning intervals, becomes a segment of a sinewave having a reduction in the rate of increase of deflection current at the positive and negative extremes of the deflection current relative to that at the center. At the center, the rate of change of the deflection current increases relative to the edges. This in turn creates a slight reduction in the rate of scanning of the electron beam near the left and right edges of the raster relative to the center and a reduction in the edge expansion and center compression nonlinearity. When sinewave S-correction is used to linearize the compressed central portion of the raster being scanned and the kinescope has a large deflection angle or radius of curvature, the correction becomes excessive near the right and left edges of the raster. This overcompensation results in an apparent compression of the displayed video at the right and left of the raster.